x86: add memory barriers to wrmsr

wrmsr is a special instruction which can have arbitrary system-wide
effects.  We don't want the compiler to reorder it with respect to
memory operations, so make it a memory barrier.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: xen-devel <xen-devel@lists.xensource.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/msr.h

@@ -66,7 +66,7 @@ static inline unsigned long long native_read_msr_safe(unsigned int msr,
 static inline void native_write_msr(unsigned int msr,
 				    unsigned low, unsigned high)
 {
-	asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high));
+	asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
 }
 
 static inline int native_write_msr_safe(unsigned int msr,
@@ -81,7 +81,8 @@ static inline int native_write_msr_safe(unsigned int msr,
 		     _ASM_EXTABLE(2b, 3b)
 		     : "=a" (err)
 		     : "c" (msr), "0" (low), "d" (high),
-		     "i" (-EFAULT));
+		       "i" (-EFAULT)
+		     : "memory");
 	return err;
 }